The famous "superconductor monorail" was a wonderful visual demonstration of what superconductors do to magnets. At first, magnets are repelled by the superconductor. As the temperature drops, a superconducting material unceremoniously kicks magnetic fields out of its territory. If a magnet is forced close to a superconductor, though, it gets 'stuck' there, and so can be forced to stay in one place. This is known as 'flux pinning.' We know what it looks like from the outside. The magnet hovers over the superconductor at a fixed distance, staying there even as the superconductor is moved around. In the case of the monorail, give it a push and it speeds around the track.

But what does that look like inside the superconductor? Scientists have found magnetic flux line 'avalanches' happening inside superconductors.

Here's the famous superconducting monorail, in case you haven't seen it:

What does this look like from the inside? Scientists have actually gotten to know what 'flux pinning' does to the inside of a superconductor, and taken pictures of how it looks. The secret to flux pinning is the fact that, with a perfect superconductor, it wouldn't happen. Magnetic flux lines shouldn't be able to penetrate the superconductor at all. A magnet should simply be repelled until its field and the superconductor have nothing to do with each other. But every superconductor has flaws. Those flaws allow the flux lines to penetrate. The 'flux' lines are tiny breaks or imperfections in the superconductor. Since the flux lines are surrounded by more perfect parts, the magnet doesn't wobble around, and is locked in place. But as the breaks happen, they precipitate more tiny breaks, like a slowly spreading cracks in a pane of glass.

Scientists have been photographing thin films of superconductors as they are exposed to magnetic fields, and have been able to capture not only the breaks themselves, but the process as they evolve. Inside of a superconductor in which the flux lines form a 'Christmas tree' formation the breaks slowly climb and spread out from each other through the the superconductor, looking like branches spreading out from a central trunk.

A more dynamic model of flux lines is the avalanche. Scientists have observed 'flux avalanches,' as the magnetic flux lines breaks through the superconductor in massive spreading waves. This is more a shattering of glass than a slow growing crack. As the magnetic field is forced close, flux lines penetrate through the superconductor suddenly, in large groups. Eventually the overall formation forms a 'path' for the magnetic field from one side of the superconductor to the other. Outside, it looks like order is imposed on the system, with the chaotically levitating magnet being 'fixed' in place. Inside, massive and disorganized breaks rip through the supposed perfection of the superconductor. There's always something going on beneath the surface.